As is known, urea is normally produced on an industrial scale via a direct biphasic reaction of ammonia and carbon dioxide under relatively high temperature and relatively high pressure conditions.
A typical urea synthesis reactor of a urea plant is fed with an essentially gaseous stream of carbon dioxide and an essentially liquid stream of ammonia/ammonium carbamate. The reagents are fed into the reactor from below, through a bottom part of the reactor and via respective distributors.
FIG. 1 schematically shows, in a simplified manner, the lower part of a typical urea synthesis reactor 1 of one known type.
In general, the reactor 1 extends along a vertical axis A and comprises a casing 2 internally defining a reaction chamber 3; the casing 2 has a basically cylindrical main portion 4 and a dome-shaped bottom portion 5 (in particular, substantially hemispherical). The bottom portion 5 is joined to the main portion 4 along an essentially circular peripheral edge 9, which lies on a plane substantially orthogonal to axis A.
The casing 2 is supported by a support frame 10, mechanically connected, in particular, to the bottom portion 5.
The reagents (carbon dioxide and ammonia/ammonium carbamate) are fed into the reactor 1 through respective dedicated distributors 15 and 16.
The distributors 15 and 16 are constituted by respective substantially vertical tubular elements, arranged to pass through the wall of the reactor 1, and precisely of the bottom portion 5, in respective openings made in said wall.
The tubular elements constituting the distributors 15 and 16 project upwards from the wall of the bottom portion 5 and form the so-called drilled pipes: each tubular element has a closed free end (located inside the reactor 1) and a plurality of lateral through holes, made in the lateral wall of the tubular element for a certain longitudinal length close to the free end.
The distributors 15 and 16 are placed in an eccentric position, for example diametrically opposed with respect to the central axis A of the reactor 1 and, in any case, generally on opposite sides of a vertical center-plane passing through the axis A of the reactor 1.
The distributors 15 and 16 constitute the end parts of respective inlet tubes bent in an elbow-shape: outside of the reactor, each tube comprises an elbow-shaped bend that connects the vertical tubular element with a horizontal section that passes through the frame 10.
A reactor with a reagent feed system such as this known type has certain drawbacks.
Apart from the related constructional complexity, mainly due to the use of elbow-shaped inlet tubes (which must normally be forged) and the need to perforate a bottom portion of the reactor, the largest drawback consists in that the vertical arrangement of the distributors does not allow uniform distribution of the reagents over the entire cross-section of the reactor.
In particular, the carbon dioxide, which due to its density constitutes the light phase in the urea synthesis reaction, tends to form a vertical column above the related distributor, in this way strongly limiting reaction kinetics, which would instead be favoured by the uniform distribution of carbon dioxide in small bubbles over the entire cross-section of the reactor. Even the distribution of the (liquid) heavy phase, formed of ammonia and ammonium carbamate, is not entirely satisfactory.
This unsatisfactory distribution of reagents, in particular of carbon dioxide, causes part of the volume of the reactor to remain unused (due to poor contact between the two phases), especially in the reactor's reagent inlet zone; this volume could instead be very productive as the concentration of the reagents is at its maximum here.